Light-emitting element

ABSTRACT

A light-emitting element, a light-emitting element unit and a light-emitting element package are provided, which are each reduced in reflection loss and intra-film light absorption by suppressing multiple light reflection in a transparent electrode layer and hence have higher luminance. The light-emitting element  1  includes a substrate  2 , an n-type nitride semiconductor layer  3 , a light-emitting layer  4 , a p-type nitride semiconductor layer  5 , a transparent electrode layer  6  and a reflective electrode layer  7 , and the transparent electrode layer  6  has a thickness T satisfying the following expression (1): 
     
       
         
           
             
               
                 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.30 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   T 
                   ≤ 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.45 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     wherein λ is the light-emitting wavelength of the light-emitting element  4 , and n is the refractive index of the transparent electrode layer  6.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. application Ser. No. 14/966,535, filed onDec. 11, 2015, and allowed on May 1, 2017, which was a Continuation ofU.S. application Ser. No. 14/147,639, filed on Jan. 6, 2014, and issuedon Jan. 5, 2016 as U.S. Pat. No. 9,231,162, which was a Continuation ofU.S. application Ser. No. 13/350,013, filed on Jan. 13, 2012, and issuedon Feb. 18, 2014 as U.S. Pat. No. 8,653,551. Furthermore, thisapplication claims the benefit of priority of Japanese PatentApplication No. 2011-006065, filed on Jan. 14, 2011. The disclosures ofthese prior U.S. and foreign applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a light-emitting element, alight-emitting element unit including the light-emitting element, and alight-emitting element package including the light-emitting element unitand a resin package encapsulating the light-emitting element unit.

Description of the Related Art

One of prior-art semiconductor light-emitting elements is disclosed, forexample, in JP-2008-263130A.

A semiconductor light-emitting element disclosed in FIG. 4 ofJP-2008-263130A includes a sapphire substrate through which light isextracted, a light-emitting layer, a transparent electrode and areflective metal layer.

When the light-emitting layer emits light, the light is mostly extractedthrough the sapphire substrate, but partly passes through thetransparent electrode to be reflected on the reflective metal layer, andfurther passes through the transparent electrode to be extracted throughthe sapphire substrate.

In the prior-art semiconductor light-emitting element, therefore, thelight emitted from the light-emitting layer is extracted at a lowerlight extracting efficiency, making it difficult to provide a sufficientluminance.

SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to provide alight-emitting element having a higher luminance.

According to the present invention, there is provided a light-emittingelement having a front surface and a back surface and adapted to extractlight from the back surface with the front surface facing down, thelight-emitting element including: a substrate transparent to alight-emitting wavelength λ and defining the back surface; an n-typenitride semiconductor layer provided on the substrate; a light-emittinglayer which is provided on the n-type nitride semiconductor layer andemits light at the light-emitting wavelength λ; a p-type nitridesemiconductor layer provided on the light-emitting layer; a transparentelectrode layer which is provided on the p-type nitride semiconductorlayer and is transparent to the light-emitting wavelength λ of thelight-emitting layer; and a reflective electrode layer which is providedon the transparent electrode layer and reflects light passing throughthe transparent electrode layer toward the back surface; wherein thetransparent electrode layer has a thickness T satisfying the followingexpression (1):

$\begin{matrix}{{\frac{3\lambda}{4n} + {0.30 \times \left( \frac{\lambda}{4n} \right)}} \leq T \leq {\frac{3\lambda}{4n} + {0.45 \times \left( \frac{\lambda}{4n} \right)}}} & (1)\end{matrix}$

wherein λ is the light-emitting wavelength of the light-emittingelement, and n is the refractive index of the transparent electrodelayer.

The term “light-emitting wavelength λ” means the peak wavelength of thespectrum of the light emitted from the light-emitting layer. Theexpression “transparent to the light-emitting wavelength λ” means thatthe transmittance at the light-emitting wavelength λ is, for example,not less than 60%.

It is generally considered that, if the thickness of the transparentelectrode layer is equal to an integer multiple of λ/4n (wherein λ isthe light-emitting wavelength and n is the refractive index of thetransparent electrode layer), the reflectance of an interface betweenthe transparent electrode layer and the p-type nitride semiconductorlayer can be reduced. Therefore, it is a conventional practice tooptically design the transparent electrode layer so that the thicknessthereof is equal to an integer multiple of λ/4n. The design is based ona precondition that the light reflected on the reflective electrodelayer includes only a perpendicular light component that is incidentperpendicularly on the transparent electrode layer.

In reality, however, the light reflected on the reflective electrodelayer includes not only the perpendicular light component but also anoblique light component that is incident obliquely on the transparentelectrode layer. Therefore, even if the transparent electrode layer isdesigned to have a thickness equal to an integer multiple of λ/4n, theoblique light component is reflected on the interface between thetransparent electrode layer and the p-type nitride semiconductor layer,and the reflected light is further reflected in the transparentelectrode layer multiple times. That is, there is a limitation inimproving the light extracting efficiency simply by designing thetransparent electrode layer so that the thickness of the transparentelectrode layer is equal to an integer multiple of λ/4n.

According to the present invention, the thickness T of the transparentelectrode layer satisfies the above expression (1). Therefore, thereflection of the light on the interface between the transparentelectrode layer and the p-type nitride semiconductor layer can bereduced as compared with the case in which the transparent electrodelayer is designed to have a thickness equal to an integer multiple ofλ/4n. As a result, multiple light reflection in the transparentelectrode layer can be reduced to improve the light extractingefficiency. Thus, the light-emitting element has a higher luminance.

The transparent electrode layer preferably includes a first electrodesublayer provided on the p-type nitride semiconductor layer in contactwith the p-type nitride semiconductor layer and having a first thicknesst₁, and a second electrode sublayer provided on the first electrodesublayer and having a second thickness t₂ that is greater than the firstthickness t₁.

The transparent electrode layer is formed, for example, by depositing anelectrode material imparted with a predetermined level of energy on thep-type nitride semiconductor layer. Therefore, an excessively highenergy level is disadvantageous, because the p-type nitridesemiconductor layer is likely to be significantly damaged.

The damage to the p-type nitride semiconductor layer can be reduced bydepositing an electrode material at a lower energy level to form thefirst electrode sublayer in contact with the p-type nitridesemiconductor layer. If the transparent electrode layer included onlythe first electrode sublayer formed by the deposition at the lowerenergy level, however, the transparent electrode layer would have apoorer quality with a lower density. Therefore, the first electrodesublayer is first formed as having a smaller thickness t₁ and then thesecond electrode sublayer is formed as having a thickness t₂ bydepositing an electrode material at a higher energy level to adjust theoverall thickness T of the transparent electrode layer, whereby thequality of the entire transparent electrode layer can be improved.

The first electrode sublayer and the second electrode sublayer may havedifferent light absorbances.

The transparent electrode layer preferably comprises ITO or ZnO. Thelight-emitting wavelength λ is preferably 450 nm.

The reflective electrode layer preferably comprises an alloy comprisingsilver, a platinum group metal and copper, and is preferably provided onthe transparent electrode layer in contact with the transparentelectrode layer.

The interface between the transparent electrode layer and the reflectiveelectrode layer of the alloy comprising silver, the platinum group metaland copper has a proper light reflecting ability, which is comparable tothat of an interface between the reflective electrode layer and aninsulative layer which may be provided between the transparent electrodelayer and the reflective electrode layer.

Since the reflective electrode layer is provided on the transparentelectrode layer in contact with the transparent electrode layer, heatgenerated by the emission of the light from the light-emitting layer isconducted directly to the reflective electrode layer from thetransparent electrode layer to be thereby efficiently released from thereflective electrode layer to the outside of the light-emitting element.

This improves the heat releasing efficiency and the light extractingefficiency.

The reflective electrode layer is preferably provided in the samepattern as the transparent electrode layer on the transparent electrodelayer, and a surface of the reflective electrode layer opposed to thetransparent electrode layer is preferably entirely kept in contact withthe transparent electrode layer.

With this arrangement, the reflective electrode layer, the transparentelectrode layer and contact portions of these layers completely overlapeach other as seen in a stacking direction (in a substrate thicknesswisedirection) in which these layers are stacked. That is, the reflectiveelectrode layer and the transparent electrode layer have nonon-overlapping portions and, therefore, are free from irregularitieswhich may otherwise occur due to the non-overlapping portions.

Thus, the light emitted from the light-emitting layer and passingthrough the transparent electrode layer can be reliably incident on theinterface between the transparent electrode layer and the reflectiveelectrode layer, and efficiently reflected on the interface. Thereflective electrode layer and the transparent electrode layer have thesame size as seen in the stacking direction, so that the interface(reflective surface) between the transparent electrode layer and thereflective electrode layer has the greatest possible size. Thus, thelight passing through the transparent electrode layer can be efficientlyreflected on the interface. This further improves the light extractingefficiency.

In the absence of the irregularities described above, a connectionsurface of the light-emitting element to be connected to an externalwiring element is flat, and a connection area between the light-emittingelement and the wiring element can be increased. This further improvesthe efficiency of heat release from the light-emitting element to thewiring element.

In the light-emitting element, no insulative layer is preferablyprovided between the transparent electrode layer and the reflectiveelectrode layer.

According to this arrangement, the reflective electrode layer of thealloy permits proper light reflection on the interface between thereflective electrode layer and the transparent electrode layer evenwithout the provision of the insulative layer. In the absence of theinsulative layer, reduction in heat releasing efficiency and lightextracting efficiency can be prevented which may otherwise occur due tothe insulative layer.

The platinum group metal is preferably platinum or palladium. Thesubstrate preferably comprises sapphire, GaN or SiC.

The substrate preferably has a plurality of projections provided on acontact surface thereof kept in contact with the n-type nitridesemiconductor layer.

With this arrangement, light traveling from the n-type nitridesemiconductor layer toward the substrate is substantially prevented frombeing reflected on the contact surface toward the n-type nitridesemiconductor layer. This correspondingly improves the light extractingefficiency.

The projections are preferably discretely arranged. In this case, theprojections may be arranged in a matrix array, or may be arranged instaggered relation.

The light-emitting layer preferably comprises In.

The light-emitting element preferably further includes a connectionlayer comprising silver, solder or AuSn and provided on the reflectiveelectrode layer. This makes it possible to provide a light-emittingelement unit which includes the light-emitting element and a wiringelement connected to the connection layer to apply a voltage from thewiring element to the light-emitting element.

The light-emitting element preferably further includes an n-typeelectrode layer which connects the wiring element to the n-type nitridesemiconductor layer. With this arrangement, the light is emitted fromthe light-emitting layer by applying the voltage between the n-typeelectrode layer and the reflective electrode layer.

The light-emitting element preferably further includes an isolationinsulative layer which isolates and insulates the n-type electrode layerand the reflective electrode layer from each other.

A light-emitting element package is also provided, which includes thelight-emitting element unit and a resin package encapsulating thelight-emitting element unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a light-emitting elementaccording to one embodiment of the present invention.

FIG. 2 is a schematic plan view of the light-emitting element.

FIG. 3 is a schematic perspective view of the light-emitting element.

FIG. 4A is a schematic perspective view showing an exemplary structureof a substrate.

FIG. 4B is a schematic perspective view showing another exemplarystructure of the substrate.

FIG. 5A is a schematic sectional view showing a method of producing thelight-emitting element shown in FIG. 1.

FIG. 5B is a schematic sectional view showing a step subsequent to thestep shown in FIG. 5A.

FIG. 5C is a schematic sectional view showing a step subsequent to thestep shown in FIG. 5B.

FIG. 5D is a schematic sectional view showing a step subsequent to thestep shown in FIG. 5C.

FIG. 5E is a schematic sectional view showing a step subsequent to thestep shown in FIG. 5D.

FIG. 5F is a schematic sectional view showing a step subsequent to thestep shown in FIG. 5E.

FIG. 5G is a schematic sectional view showing a step subsequent to thestep shown in FIG. 5F.

FIG. 6 is a schematic sectional view of a wiring element.

FIG. 7 is a schematic plan view of the wiring element.

FIG. 8 is a schematic sectional view of a light-emitting elementpackage.

FIG. 9 is a schematic plan view of the wiring element, illustrating theconnection state of the wiring element and the light-emitting element.

FIG. 10 is a schematic perspective view of the light-emitting elementpackage.

FIG. 11 is a graph showing a relationship between the luminance changeratio of the light-emitting element and the thickness of an ITO film.

FIG. 12 is a diagram showing a variation of the light-emitting elementshown in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the present invention will hereinafter be described indetail with reference to the attached drawings.

FIG. 1 is a schematic sectional view of a light-emitting elementaccording to one embodiment of the present invention. FIG. 2 is aschematic plan view of the light-emitting element. FIG. 3 is a schematicperspective view of the light-emitting element.

The light-emitting element 1 includes a substrate 2, an n-type nitridesemiconductor layer 3, a light-emitting layer 4, a p-type nitridesemiconductor layer 5, a transparent electrode layer 6, a reflectiveelectrode layer 7, an n-type electrode layer 8, an isolation insulativelayer 9 and connection layers 10.

The n-type nitride semiconductor layer 3, the light-emitting layer 4,the p-type nitride semiconductor layer 5, the transparent electrodelayer 6, the reflective electrode layer 7, the n-type electrode layer 8,the isolation insulative layer 9 and the connection layers 10 areprovided on the substrate 2.

The substrate 2 is made of a material (e.g., sapphire, GaN or SiC)transparent to a light-emitting wavelength λ (e.g., 450 nm) of thelight-emitting layer 4 at which light is emitted from the light-emittinglayer 4. The substrate 2 has a thickness of, for example, 400 μm. Thesubstrate 2 has a back surface 2A defined by a lower surface thereof inFIG. 1, and a front surface 2B defined by an upper surface thereof inFIG. 1. The back surface 2A serves as a light extracting surface fromwhich the light is extracted, and defines a back surface of thelight-emitting element 1. The front surface 2B serves as a contactsurface of the substrate 2 kept in contact with the n-type nitridesemiconductor layer 3.

The n-type nitride semiconductor layer 3 is provided on the substrate 2.The n-type nitride semiconductor layer 3 covers the entire front surface2B of the substrate 2. The n-type nitride semiconductor layer 3 is madeof n-type GaN, and is transparent to the light-emitting wavelength λ ofthe light-emitting layer 4. The n-type nitride semiconductor layer 3 hasa back surface 3A defined by a lower surface thereof covering the frontsurface 2B of the substrate 2 in FIG. 1, and a front surface 3B definedby an upper surface thereof opposite from the back surface 3A in FIG. 1.A left portion of the front surface 3B in FIG. 1 projects, so that thefront surface 3B has a step.

The light-emitting layer 4 is provided on the n-type nitridesemiconductor layer 3. The light-emitting layer 4 is formed by a dryetching method. The light-emitting layer 4 covers the left projectingportion of the front surface 3B of the n-type nitride semiconductorlayer 3 in FIG. 1. The light-emitting layer 4 is made of anIn-containing nitride semiconductor (e.g., InGaN). The light-emittinglayer 4 has a back surface 4A defined by a lower surface thereofcovering the left portion of the front surface 3B of the n-type nitridesemiconductor layer 3 in FIG. 1, and a front surface 4B defined by anupper surface thereof opposite from the back surface 4A in FIG. 1.

The p-type nitride semiconductor layer 5 is provided on thelight-emitting layer 4. The p-type nitride semiconductor layer 5 coversthe entire front surface 4B of the light-emitting layer 4. The p-typenitride semiconductor layer 5 is formed together with the light-emittinglayer 4 by the dry etching method. The p-type nitride semiconductorlayer 5 is made of p-type GaN, and is transparent to the light-emittingwavelength λ of the light-emitting layer 4. The total thickness of then-type nitride semiconductor layer 3, the light-emitting layer 4 and thep-type nitride semiconductor layer 5 is, for example, 6.5 μm at amaximum. The p-type nitride semiconductor layer 5 has a back surface 5Adefined by a lower surface thereof covering the front surface 4B of thelight-emitting layer 4 in FIG. 1, and a front surface 5B defined by anupper surface thereof opposite from the back surface 5A in FIG. 1.

The transparent electrode layer 6 is provided on the p-type nitridesemiconductor layer 5. The transparent electrode layer 6 coverssubstantially the entire front surface 5B of the p-type nitridesemiconductor layer 5. The transparent electrode layer 6 is formed, forexample, by a lift-off method. The transparent electrode layer 6 is madeof ZnO (zinc oxide) or ITO (indium tin oxide), and is transparent to thelight-emitting wavelength λ of the light-emitting layer 4. In thisembodiment, the transparent electrode layer 6 is made of ITO. Thetransparent electrode layer 6 has a thickness T which satisfies thefollowing expression (1):

$\begin{matrix}{{\frac{3\lambda}{4n} + {0.30 \times \left( \frac{\lambda}{4n} \right)}} \leq T \leq {\frac{3\lambda}{4n} + {0.45 \times \left( \frac{\lambda}{4n} \right)}}} & (1)\end{matrix}$

wherein λ is the light-emitting wavelength of the light-emitting layer4, and n is the refractive index of the transparent electrode layer 6.

For example, ITO has a refractive index n of 2.0 and, where thelight-emitting wavelength λ of the light-emitting layer 4 is 450 nm, thethickness T of the transparent electrode layer 6 is about 1850 Å toabout 1950 Å.

The transparent electrode layer 6 has a back surface 6A defined by alower surface thereof covering the front surface 5B of the p-typenitride semiconductor layer 5 in FIG. 1, and a front surface 6B definedby an upper surface thereof opposite from the back surface 6A in FIG. 1.

The reflective electrode layer 7 is provided in the same pattern as thetransparent electrode layer 6 on the transparent electrode layer 6. Thereflective electrode layer 7 covers the entire front surface 6B of thetransparent electrode layer 6 so as not to protrude from the frontsurface 6B in FIG. 1. The reflective electrode layer 7 has a backsurface 7A defined by a lower surface thereof covering the front surface6B of the transparent electrode layer 6 in FIG. 1, and a front surface7B defined by an upper surface thereof opposite from the back surface7A. The back surface 7A of the reflective electrode layer 7 serves as anopposed surface which is opposed to the front surface 6B of thetransparent electrode layer 6, and is entirely kept in contact (surfacecontact) with the front surface 6B of the transparent electrode layer 6.Therefore, nothing (e.g., no insulative layer) is present between thetransparent electrode layer 6 and the reflective electrode layer 7.

In this embodiment, the reflective electrode layer 7 is made of an alloycontaining silver, a platinum group metal and copper. Usable examples ofthe platinum group metal include platinum and palladium. The proportionsof silver, the platinum group metal and copper in the alloy are about98%, about 1% and about 1%, respectively. The reflective electrode layer7 typically has a thickness of, for example, 50 nm to 500 nm, preferably350 nm.

The n-type electrode layer 8 is provided on a right portion of the frontsurface 3B of the n-type nitride semiconductor layer 3 in FIG. 1. Then-type electrode layer 8 is provided on a portion of the front surface3B of the n-type nitride semiconductor layer 3 which is recessed towardthe rear surface 3A to form the step described above. The n-typeelectrode layer 8 is made of Al and Cr. In this embodiment, the n-typeelectrode layer 8 is formed by forming an Al sublayer in contact withthe n-type nitride semiconductor layer 3 and then forming a Cr sublayeron the Al sublayer. The n-type electrode layer 8 has a thickness of, forexample, about 26000 Å. The n-type electrode layer 8 has a back surface8A defined by a surface thereof in contact with the front surface 3B ofthe n-type nitride semiconductor layer 3 in FIG. 1, and a front surface8B defined by a surface thereof opposite from the back surface 8A.

Side surfaces of the n-type nitride semiconductor layer 3, thelight-emitting layer 4, the p-type nitride semiconductor layer 5, thetransparent electrode layer 6 and the reflective electrode layer 7 arecovered with the isolation insulative layer 9 which is made of, forexample, SiO₂. Thus, the light-emitting layer 4, the p-type nitridesemiconductor layer 5, the transparent electrode layer 6 and thereflective electrode layer 7 are isolated and insulated from the n-typeelectrode layer 8. Sin, AN, Al₂O₃ or SiON may be used instead of SiO₂ asa material for the isolation insulative layer 9. The isolationinsulative layer 9 has a thickness of 500 Å to 50000 Å, for example,1000 Å. In FIGS. 2 and 3, the isolation insulative layer 9 is not shown.

The connection layers 10 are respectively provided on the reflectiveelectrode layer 7 and the n-type electrode layer 8. The connectionlayers 10 each comprise, for example, Ag, Ti or Pt, or an alloy of anyof these metals. The connection layers 10 may each comprise solder orAuSn. The connection layers 10 may each include a Pt sublayer forsuppression of diffusion of the connection layer materials from theconnection layers 10 to the reflective electrode layer 7 and the n-typeelectrode layer 8. In this embodiment, the connection layers 10 eachinclude a Ti sublayer, a Pt sublayer and an AuSn sublayer stacked inthis order from the reflective electrode layer 7 and the n-typeelectrode layer 8. The connection layers 10 each have a back surface 10Adefined by a lower surface thereof in contact with the reflectiveelectrode layer 7 or the n-type electrode layer 8 in FIG. 1, and a frontsurface 10B defined by an upper surface thereof opposite from the backsurface 10A in FIG. 1. The front surfaces 10B of the connection layers10 serve as a front surface of the light-emitting element 1.

The front surface 10B of the connection layer 10 contacting thereflective electrode layer 7 serves as a p-type electrode portion 12,and the front surface 10B of the connection layer 10 contacting then-type electrode layer 8 serves as an n-type electrode portion 13. Thep-type electrode portion 12 and the n-type electrode portion 13respectively have flat surfaces, which are located at the same heightlevel to be flush with each other (also see FIG. 3). Since thereflective electrode layer 7 and the n-type electrode layer 8 areisolated and insulated from each other by the isolation insulative layer9 as described above, the p-type electrode portion 12 of the connectionlayer 10 on the reflective electrode layer 7 and the n-type electrodeportion 13 of the connection layer 10 on the n-type electrode layer 8are isolated and insulated from each other by the isolation insulativelayer 9.

As seen in plan, the p-type nitride semiconductor layer 5, thetransparent electrode layer 6, the reflective electrode layer 7 and thep-type electrode portion 12 of the connection layer 10 each have, forexample, a generally E-shape, and the n-type electrode layer 8 and then-type electrode portion 13 of the connection layer 10 each have agenerally I-shape (see FIG. 2). The n-type electrode layer 8 has twoextension portions 8C which respectively extend into two spaces definedby the E-shapes of the p-type nitride semiconductor layer 5, thetransparent electrode layer 6, the reflective electrode layer 7 and theconnection layer 10 (p-type electrode portion 12) (see FIGS. 2 and 3).

When a forward voltage is applied between the p-type electrode portion12 and the n-type electrode portion 13, the light is emitted from thelight-emitting layer 4 at a light-emitting wavelength λ of 440 nm to 460nm in the light-emitting element 1. The light passes through the n-typenitride semiconductor layer 3 and the substrate 2 in this order to beextracted from the back surface 2A of the substrate 2. Light travelingfrom the light-emitting layer 4 toward the p-type nitride semiconductorlayer 5 passes through the p-type nitride semiconductor layer 5 and thetransparent electrode layer 6 in this order to be reflected on aninterface between the transparent electrode layer 6 and the reflectiveelectrode layer 7. The reflected light passes through the transparentelectrode layer 6, the p-type nitride semiconductor layer 5, thelight-emitting layer 4, the n-type nitride semiconductor layer 3 and thesubstrate 2 in this order to be extracted from the back surface 2A ofthe substrate 2.

A plurality of projections 11 are provided on the front surface 2B ofthe substrate 2 as projecting toward the n-type nitride semiconductorlayer 3.

FIGS. 4A and 4B are schematic perspective views showing exemplarystructures of the substrate.

The projections 11 are discretely arranged. More specifically, theprojections 11 may be spaced from each other to be arranged in a matrixarray (see FIG. 4A), or may be arranged in staggered relation (see FIG.4B). The projections 11 are each made of SiN.

With the provision of the projections 11 of SiN, light rays incident atdifferent angles are substantially prevented from being totallyreflected on the front surface 2B of the substrate 2. Thus, light raystraveling from the n-type nitride semiconductor layer 3 toward thesubstrate 2 are substantially prevented from being reflected on theinterface between the n-type nitride semiconductor layer 3 and thesubstrate 2 toward the n-type nitride semiconductor layer 3. Thisimproves the light extracting efficiency.

FIGS. 5A to 5H are schematic sectional views showing a method ofproducing the light-emitting element shown in FIG. 1.

First, a substrate 2 is prepared as shown in FIG. 5A.

Then, a layer of SiN (SiN layer) is formed on a front surface 2B of thesubstrate 2. The SiN layer is etched with the use of a resist pattern(not shown) as a mask to be thereby divided into a plurality ofprojections 11 as shown in FIG. 5B.

In turn, a layer of n-type GaN (n-GaN layer) is formed over the frontsurface 2B of the substrate 2. The n-GaN layer serves as an n-typenitride semiconductor layer 3 on the substrate 2, and covers all theprojections 11.

Subsequently, as shown in FIG. 5C, an In-containing nitridesemiconductor layer (e.g., In_(x)Ga_(1-x)N layer) is formed on a frontsurface 3B of the n-type nitride semiconductor layer 3. This layerserves as a light-emitting layer 4 on the n-type nitride semiconductorlayer 3. The light-emitting wavelength λ of the light-emitting layer 4is adjusted to 440 nm to 460 nm by controlling the composition ratio ofIn and Ga.

Then, a layer of p-type GaN (p-GaN layer) is formed as a p-type nitridesemiconductor layer 5 on a front surface 4B of the light-emitting layer4. A p-AlGaN layer containing Al or a layered structure including ap-GaN sublayer and a p-AlGaN sublayer may be employed as the p-typenitride semiconductor layer 5.

In turn, a resist pattern (not shown) having an opening in a region tobe formed with a transparent electrode layer 6 is formed on the p-typenitride semiconductor layer 5. Subsequently, an ITO material isdeposited on the p-type nitride semiconductor layer 5 via the resistpattern, for example, by a sputtering method. Then, an unnecessaryportion of the ITO material is lifted off together with the resistpattern. Thus, a layer of ITO (ITO layer) is formed on a selectedportion of a front surface 5B of the p-type nitride semiconductor layer5 as shown in FIG. 5D. The ITO layer serves as a transparent electrodelayer 6. In the formation of the transparent electrode layer 6, a heattreatment may be performed to improve electrical connection and adhesionbetween the p-type nitride semiconductor layer 5 and the transparentelectrode layer 6. In this case, the heat treatment is performed, forexample, at a temperature of 500° C. to 700° C.

Subsequently, a layer of an alloy containing silver, a platinum groupmetal and copper (alloy layer) is formed over the front surface 6B ofthe transparent electrode layer 6 and the front surface 5B of the p-typenitride semiconductor layer 5, and etched with the use of a resistpattern (not shown) as a mask, whereby a reflective electrode layer 7 isformed in the same pattern as the transparent electrode layer 6 on thetransparent electrode layer 6 as shown in FIG. 5D.

Then, parts of the p-type nitride semiconductor layer 5, thelight-emitting layer 4 and the n-type nitride semiconductor layer 3 areselectively etched off as shown in FIG. 5E with the use of a resistpattern (not shown) as a mask.

In turn, as shown in FIG. 5F, an n-type electrode layer 8 is formed onthe front surface 3B of the n-type nitride semiconductor layer 3 by alift-off method using a resist pattern (not shown). The n-type electrodelayer 8 may be made of Al, or may have a layered structure including aTi sublayer and an Al sublayer. In the formation of the n-type electrodelayer 8, a heat treatment may be performed for improvement of adhesionand electrical connection between the n-type electrode layer 8 and then-type nitride semiconductor layer 3.

Subsequently, as shown in FIG. 5G, an isolation insulative layer 9 ofSiO₂ is formed. The isolation insulative layer 9 is formed so as tocover a part of the front surface 3B of the n-type nitride semiconductorlayer 3 to be located adjacent a p-type electrode portion 12 (see FIG.1), side surfaces of the light-emitting layer 4, the p-type nitridesemiconductor layer 5, the transparent electrode layer 6 and thereflective electrode layer 7, and a part of a front surface 7B of thereflective electrode layer 7. The formation of the isolation insulativelayer 9 is achieved by a lift-off method using a resist pattern (notshown) or an etch-off method. SiN, AlN, Al₂O₃ or SiON may be usedinstead of SiO₂ as the material for the isolation insulative layer 9.

Then, connection layers 10 are formed on the front surface 7B of thereflective electrode layer 7 and the front surface 8B of the n-typeelectrode layer 8 by a lift-off method using a resist pattern (notshown). In this embodiment, the connection layers 10 each include asublayer of AuSn (AuSn sublayer). The connection layers 10 each furtherinclude a Pt sublayer for protection of the reflective electrode layer 7and the n-type electrode layer 8 from diffusion of AuSn from the AuSnsublayers. The connection layers 10 each further include a Ti sublayerfor improvement adhesion between the connection layer 10 and thereflective electrode layer 7 and between the connection layer 10 and then-type electrode layer 8. In this embodiment, therefore, the Tisublayer, the Pt sublayer and the AuSn sublayer are stacked in thisorder on each of the reflective electrode layer 7 and the n-typeelectrode layer 8 to form the connection layer 10.

A structure shown in FIG. 1 is formed by the process sequence describedabove.

In practice, the process sequence described above with reference toFIGS. 5A to 5G is performed as a semiconductor wafer process for forminga plurality of light-emitting elements 1 each having the structure shownin FIG. 1 on a wafer. After this process, the thickness of the wafer isadjusted to, for example, 300 μm by a grinding/polishing process. Then,the wafer is divided into the plurality of light-emitting elements 1(chips) by an element isolating process (scribing/breaking process).

FIG. 6 is a schematic sectional view of a wiring element. Thelight-emitting element 1 is connected to the wiring element 20 via theconnection layers 10 thereof.

Referring to FIG. 6, the wiring element 20 includes a base substrate 21,an insulative layer 22, electrode layers 23 and connection layers 24.

The base substrate 21 is made of, for example, Si, and has a thicknessof, for example, 130 μm. The insulative layer 22 is made of, forexample, SiO₂, and covers the entire front surface (upper surface inFIG. 6) of the base substrate 21. The insulative layer 22 has athickness of, for example, 1000 Å. A back surface (lower surface in FIG.6) of the base substrate 21 defines a back surface of the wiring element20.

The electrode layers 23 are made of, for example, Al, and each have athickness of, for example, 25000 Å. Two electrode layers 23 are providedin two positions on the insulative layer 22 in laterally spaced relationin FIG. 6.

The connection layers 24 are respectively provided on the electrodelayers 23. The connection layers 24 each have a double layered structureincluding a Ti sublayer 25 provided closer to the base substrate 21 andan Au sublayer 26 provided on the Ti sublayer 25. The Ti sublayer 25 ismade of Ti, and has a thickness of, for example, 1000 Å. The Au sublayer26 is made of Au, and has a thickness of, for example, 10000 Å. Theconnection layers 24 each have a front surface 24A defined by a surface(upper surface in FIG. 6) thereof opposite from a surface contacting theelectrode layer 23. The front surfaces 24A of the connection layers 24are flat, and define a front surface of the wiring element 20.

FIG. 7 is a schematic plan view of the wiring element.

As seen in plan in FIG. 7, the connection layers 24 include a generallyE-shaped connection layer conformal to the p-type electrode portion 12and a generally I-shaped connection layer conformal to the n-typeelectrode portion 13 in combination (also see FIG. 2).

FIG. 8 is a schematic sectional view of a light-emitting elementpackage. FIG. 9 is a schematic plan view of the wiring element,illustrating the connection state of the wiring element and thelight-emitting element. FIG. 10 is a schematic perspective view of thelight-emitting element package. The light-emitting package 50 includesthe light-emitting element unit 30, a support board 31 and a resinpackage 40.

As shown in FIG. 6, the wiring element 20 is placed with the frontsurfaces 24A of the connection layers 24 thereof facing up. Thelight-emitting element 1 shown in FIG. 1 is held with the front surfaces10B (the p-type electrode portion 12 and the n-type electrode portion13) of the connection layers 10 thereof facing down (in an attitudeturned upside down from that shown in FIG. 1), and brought into opposedrelation to the wiring element 20 assuming an attitude of FIG. 6 fromthe above.

By moving the light-emitting element 1 toward the wiring element 20, thefront surfaces 10B of the connection layers 10 are brought into surfacecontact with the front surfaces 24A of the connection layers 24 as shownin FIG. 8. More specifically, the p-type electrode portion 12 and then-type electrode portion 13 of the connection layers 10 are brought intosurface contact with the front surfaces 24A of the left connection layer24 and the right connection layer 24, respectively, in FIG. 8. In thisstate, a heat treatment process is performed to bond the connectionlayers 10 to the connection layers 24, respectively, by fusion andsolidification. As a result, the light-emitting element 1 and the wiringelement 20 are combined together to provide the light-emitting elementunit 30.

In the completed light-emitting element unit 30, the front surfaces 10Bof the connection layers 10 are respectively laid on and bonded to thefront surfaces 24A of the connection layers 24 as indicated by hatchingin FIG. 9. The front surfaces 10B of the connection layers 10 and thefront surfaces 24A of the connection layers 24 are flat and flush witheach other and, therefore, have no portion uninvolved in the bonding.Thus, the entire front surfaces 10B of the connection layers 10 arerespectively bonded to the entire front surfaces 24A of the connectionlayers 24. The connection layers 10 are also electrically connected tothe connection layers 24, respectively, by the bonding.

Referring to FIG. 8, the light-emitting element unit 30 is connected tothe support board 31. The support board 31 includes an insulativesubstrate 32 supporting the light-emitting element unit 30, and a pairof metal leads 33 exposed along opposite edges of the insulativesubstrate 32 for electrical connection between the light-emittingelement 1 and an external element.

Where the light-emitting element unit 30 assumes a reference attitude asshown in FIG. 8, the substrate 2 of the light-emitting element 1 islocated at the uppermost position, and the base substrate 21 of thewiring element 20 is located at the lowermost position. In this state,the base substrate 21 is bonded to the insulative substrate 32 from theabove. Then, the electrode layer 23 (left electrode layer 23 in FIG. 8)on which the connection layer 24 connected to the p-type electrodeportion 12 is provided is connected to one of the leads 33 locatedadjacent this electrode layer 23 via a bonding wire 34. Further, theelectrode layer 23 (right electrode layer 23 in FIG. 8) on which theconnection layer 24 connected to the n-type electrode portion 13 isprovided is connected to the other lead 33 located adjacent thiselectrode layer 23 via a bonding wire 34.

The resin package 40 is a case filled with a resin. The light-emittingelement unit 30 is accommodated (or encapsulated) in the resin package40 for protection thereof and, in this state, fixed to the support board31. The resin package 40 has a reflective portion 40A on its side wall(opposed to the light-emitting element unit 30) to reflect light emittedfrom the light-emitting element 1 of the light-emitting element unit 30for extraction of the light.

The resin of the resin package 40 may contain a fluorescent material anda reflective material. Where the light-emitting element 1 emits bluelight, for example, a yellow fluorescent material is added to the resinto cause the light-emitting element package 50 to emit white light. Amultiplicity of such light-emitting element packages 50 may be combinedtogether for use as an lighting device such as an electric lamp, a backlight for a liquid crystal TV and a head lamp for a motor vehicle.

The light-emitting element package according to the present invention isnot limited in structure to the light-emitting element package 50, butmay be constructed such that the light-emitting element unit 30 isconnected to a pair of lead frames respectively electrically connectedto the p-type electrode portion 12 and the n-type electrode portion 13of the light-emitting element unit 30 by a flip bonding method.

In the light-emitting element 1, as described above, the light emittedfrom the light-emitting layer 4 mostly passes through the n-type nitridesemiconductor layer 3 to be extracted from the substrate 2, but partlypasses through the p-type nitride semiconductor layer 5 and thetransparent electrode layer 6 to be reflected on the interface betweenthe transparent electrode layer 6 and the reflective electrode layer 7and then extracted from the substrate 2.

Since the thickness T of the transparent electrode layer 6 of thelight-emitting element 1 satisfies the above expression (1), the lightreflection on the interface between the transparent electrode layer 6and the p-type nitride semiconductor layer 5 and on the interfacebetween the transparent electrode layer 6 and the reflective electrodelayer 7 can be reduced as compared with the case in which thetransparent electrode layer 6 is designed to have a thickness T equal toan integer multiple of λ/4n. As a result, multiple light reflection inthe transparent electrode layer 6 is suppressed to improve the lightextracting efficiency. This increases the luminance of thelight-emitting element 1. More specifically, this is explained withreference to FIG. 11.

FIG. 11 is a graph showing a relationship between the luminance changeratio of the light-emitting element and the thickness of an ITO film.

Light-emitting elements (each employing an ITO film as the transparentelectrode layer 6 and GaN films as the nitride semiconductor layers)having the same construction as the light-emitting element 1 shown inFIG. 1 were produced, and it was checked how luminance observed whenlight was emitted from the light-emitting layer at a light-emittingwavelength λ of 450 nm was changed with respect to the thickness of theITO film (ITO film thickness).

Luminance observed when the ITO film thickness was 2050 Å was used as areference luminance value (1.000 under reference conditions) by way ofexample, and the change ratio of an experiment luminance value relativeto the reference luminance value was determined for evaluation. Forexample, a luminance change ratio of 1.100 indicates that the experimentluminance value was 10% greater than the reference luminance value.

FIG. 11 indicates that, when the ITO film thickness T satisfies theabove expression (1), the experiment luminance value is 2% to 13%greater than the reference luminance value. Experiment values providingsuch a luminance increasing effect are plotted to provide an approximatecurve indicated by a broken line in FIG. 11.

The approximate curve is phase-shifted in an ITO film thicknessincreasing direction with respect to a curve (indicated by a solid linein FIG. 11) showing a change in the light transmittance of the ITO filmdetermined in consideration of only a light component incidentperpendicularly on the ITO film. In the solid line curve, the luminanceis increased as the light transmittance increases, because the multiplelight reflection in the ITO film is reduced.

That is, where not only the perpendicular light component but also anoblique light component incident obliquely on the ITO film is taken intoconsideration, an optimum value of the ITO film thickness (at a vertexof the broken line curve) is shifted in the ITO film thicknessincreasing direction.

The description made with reference to FIG. 11 is directed to a case inwhich the light-emitting wavelength λ is 450 nm, but the light-emittingwavelength λ is not limited to 450 nm in the present invention.

The interface between the transparent electrode layer 6 and thereflective electrode layer 7 of the alloy containing silver, theplatinum group metal and copper has a proper light reflecting ability,which is comparable to that of an interface between the reflectiveelectrode layer 7 and an insulative layer (not shown) which may beprovided between the transparent electrode layer 6 and the reflectiveelectrode layer 7.

Since the reflective electrode layer 7 is provided on the transparentelectrode layer 6 in contact with the transparent electrode layer 6,heat generated by the emission of the light from the light-emittinglayer 4 is conducted directly to the reflective electrode layer 7 fromthe transparent electrode layer 6 to be thereby efficiently releasedfrom the reflective electrode layer 7 to the outside of thelight-emitting element 1 (wiring element 20).

This further improves the heat releasing efficiency and the lightextracting efficiency.

The reflective electrode layer 7 is provided in the same pattern as thetransparent electrode layer 6 on the transparent electrode layer 6, andthe entire back surface 7A of the reflective electrode layer 7 opposedto the transparent electrode layer 6 is kept in contact with the frontsurface 6B of the transparent electrode layer 6. Therefore, the backsurface 7A of the reflective electrode layer 7 and the front surface 6Bof the transparent electrode layer 6 completely overlap each other, sothat the reflective electrode layer 7 and the transparent electrodelayer 6 are free from irregularities which may otherwise occur due tonon-overlapping portions thereof.

Thus, the light emitted from the light-emitting layer 4 and passingthrough the transparent electrode layer 6 can be efficiently reflectedon the interface between the transparent electrode layer 6 and thereflective electrode layer 7 to be extracted without hindrance by theirregularities. The reflective electrode layer 7 and the transparentelectrode layer 6 have the same pattern as seen in the stackingdirection, so that the interface between the transparent electrode layer6 and the reflective electrode layer 7 has the greatest possible area.Thus, the light passing through the transparent electrode layer 6 can beefficiently reflected on the interface. This further improves the lightextracting efficiency.

In the absence of the irregularities described above, the connectionsurface of the light-emitting element 1 to be connected to the externalwiring element 20 (the front surfaces 10B of the connection layers 10)is flat, and a connection area between the light-emitting element 1 andthe wiring element 20 can be increased. The connection area herein meansthe area of connection between the front surfaces 10B of the connectionlayers 10 and the front surfaces 24A of the connection layers 24 of thewiring element 20 (see FIG. 9). With the increased connection area, theefficiency of heat release from the light-emitting element 1 to thewiring element 20 can be improved.

Further, no insulative layer is provided between the transparentelectrode layer 6 and the reflective electrode layer 7. Even without theprovision of the insulative layer, the reflective electrode layer 7 ofthe aforementioned alloy permits proper light reflection on theinterface between the transparent electrode layer 6 and the reflectiveelectrode layer 7. In the absence of the insulative layer, reduction inheat release efficiency and light extracting efficiency can be preventwhich may otherwise occur due to the presence of the insulative layer.

Since the plurality of projections 11 are provided on the front surface2B of the substrate 2, the light traveling from the n-type nitridesemiconductor layer 3 toward the substrate 2 is substantially preventedfrom being reflected on the front surface 2B of the substrate 2 towardthe n-type nitride semiconductor layer 3. This correspondingly improvesthe light extracting efficiency.

While the embodiment of the present invention has thus been described,the invention may be embodied in other ways.

For example, the transparent electrode layer 6 may have a layeredstructure including a first electrode sublayer 61 provided on the p-typenitride semiconductor layer 5 in contact with the p-type nitridesemiconductor layer 5 and having a first thickness t₁, and a secondelectrode sublayer 62 provided on the first electrode sublayer 61 andhaving a second thickness t₂ greater than the first thickness t₁. Thefirst thickness t₁ is, for example, 5 Å to 500 Å, and the secondthickness t₂ is, for example, 1400 Å to 1900 Å. The first electrodesublayer 61 has a light absorbance of, for example, 0 to 5% (at alight-emitting wavelength of 450 nm), and the second electrode layer 62has a light absorbance of, for example, 0 to 2% (at a light-emittingwavelength of 450 nm).

When the transparent electrode layer 6 is formed as having the structureshown in FIG. 12 (in the step shown in FIG. 5D), the first electrodesublayer 61 is formed by deposition at a lower energy level, whereby adamage to the p-type nitride semiconductor layer 5 is reduced. Inaddition, the overall thickness T of the transparent electrode layer 6may be adjusted by forming the first electrode sublayer 61 to thesmaller thickness t₁ and then forming the second electrode sublayer 62by deposition at a higher energy level, thereby improving the quality ofthe overall transparent electrode layer 6.

The material for the reflective electrode layer 7 is not limited to thealloy containing silver, the platinum group metal and copper, but otherexamples of the material include silver (Ag) and Rh (rhodium).

The reflective electrode layer 7 is not necessarily required to be keptin direct contact with the transparent electrode layer 6, but aninsulative layer, for example, may be provided between the reflectiveelectrode layer 7 and the transparent electrode layer 6.

It should be understood that the embodiment of the present invention ismerely illustrative of the technical principles of the invention but notlimitative of the invention. The spirit and scope of the presentinvention are to be limited only by the appended claims.

What is claimed is:
 1. A light-emitting element comprising: a substratehaving a front surface and a rear surface; a first conductive layerformed on the front surface of the substrate, the first conductive layerhaving a first conductivity type; a light-emitting layer formed on thefirst conductive layer such that the first conductive layer has anexposed portion which is exposed from a region other than thelight-emitting layer; a second conductive layer formed on thelight-emitting layer, the second conductive layer having a secondconductivity type which is an opposite conductivity type to the firstconductivity type; a first electrode formed on the second conductivelayer, the first electrode electrically connected to the secondconductive layer; a second electrode formed on the exposed portion ofthe first conductive layer; the first electrode and the second electrodepartially overlapping with each other in one planar surface parallel tothe rear surface of the substrate; and the first electrode and thesecond electrode partially overlapping with each other in bothdirections of a first direction and a second direction in planar view,the first direction being parallel to one side of the substrate and thesecond direction being perpendicular to the first direction.
 2. Thelight-emitting element according to claim 1, wherein the first electrodehas at least two portions between which at least part of the secondelectrode is interposed in the second direction.
 3. The light-emittingelement according to claim 1, wherein the second electrode has at leasttwo portions between which at least part of the first electrode isinterposed in the second direction.
 4. The light-emitting elementaccording to claim 1, wherein the first electrode has a concave portionin planar view, the second electrode has a convex portion in planarview, and the convex portion of the second electrode is disposed in theconcave portion of the first electrode in planar view.
 5. Thelight-emitting element according to claim 4, wherein the first electrodehas two of the concave portions in planar view, and the second electrodehas two of the convex portions in planar view.
 6. The light-emittingelement according to claim 1, wherein the second electrode has a concaveportion in planar view, the first electrode has a convex portion inplanar view, and the convex portion of the first electrode is disposedin the concave portion of the second electrode in planar view.
 7. Thelight-emitting element according to claim 6, wherein the first electrodehas two of the convex portions in planar view, and the second electrodehas two of the concave portions in planar view.
 8. The light-emittingelement according to claim 1, wherein a distance between a surface ofthe first electrode and the rear surface of the substrate is equal to adistance between a surface of the second electrode and the rear surfaceof the substrate.
 9. The light-emitting element according to claim 1,wherein the light-emitting layer is formed on an inner region of aperipheral of the first conductive layer in planar view.
 10. Thelight-emitting element according to claim 1, wherein the first electrodeis formed on an inner region of a peripheral of the light-emitting layerin planar view.
 11. The light-emitting element according to claim 1,wherein a length of the first electrode in a third directionperpendicular to the rear surface of the substrate is shorter than alength of the second direction in the third direction.
 12. Thelight-emitting element according to claim 1, wherein the secondelectrode has a first area of a first conductive layer side and a secondarea of an opposite side to the first conductive layer side, and thefirst area is larger than the second area.